1. Field of the Invention
The present invention relates to an optical receiver to be applied to an optical communication system, in particularly, related to an optical receiver using an avalanche photodiode (APD).
2. Related Prior Art
A PON (Passive Optical Network) system is well known to perform two-way optical communication between a master station and a plurality of slave stations. Between the master station and the plural stations in the PON system, optical fibers and optical branching devices are provided. The master station sends, in one time, an instruction to the slave stations, which includes a timetable. The slave stations, based on the timetable, sequentially send upstream signals to the master station.
The avalanche photodiode (APD) is typically used an optical receiver in the master station because the APD has a photo-multiplication characteristic, and accordingly, the APD may respond to a weak optical signal. However, when a large optical signal enters the APD biased at a high voltage, the APD easily saturates, and occasionally the APD may be destroyed by the photo-carrier generated by itself. The carrier-multiplication strongly depends on the reverse bias voltage applied thereto. Thus, in the optical receiver, control of the bias voltage must be necessary such that the bias voltage is to be large for weak optical signals and to be small for large optical signals.
Self-biased optical receiver using a current feedback effect is well known, in which a resistor is connected in series between the APD and the bias voltage source. In this configuration, when the large optical signal is input, and the APD generates a large current, the voltage drop at the resistor connected in series to the APD becomes large, which reduces the bias voltage applied to the APD.
Another configuration of the optical receiver using the APD is disclosed in JP-A-2003-069500, in which the optical receiver monitors not only the intensity but also the bit error rate of the upstream signal, and adjusts the bias voltage to the APD thereby.
However, in the self-bias type optical receiver, the bias voltage is set with a time lag due to parasitic capacitance, accordingly, the optical receiver having the self-bias configuration may not follow the steep reduction/enhancement of the intensity of the optical signal sequentially received thereby.
Further, in the configuration disclosed in the Japanese patent listed above, the optical receiver there in may not follow the optical signals with a short time interval (about 1 ns), because the bit error must be measured within a substantial period, and the feedback loop has a time constant.